Juice directly from fresh fruits and vegetables is generally preferred by people, but is readily available only during growing seasons and in specific locations. Thus, juice from the fruits and vegetables must be shipped to other locations than where the fruit and vegetables are grown and the juice must be stored for later use during off-season.
To reduce shipping cost and to achieve longer storage, juices are concentrated and otherwise processed. Since consumers generally prefer the flavor, aroma, appearance and mouth feel of freshly squeezed juices, processors strive to retain such characteristics of freshly squeezed juice in the concentrate and ultimately in the reconstituted juice.
For example, orange juice is frequently concentrated to 60 to 65 weight percent sugar as sucrose (60.degree. to 65.degree. Brix) for shipping from large growing areas such as Brazil primarily to avoid the cost of shipping large quantities of water. Closer to the market, processors will dilute the concentrate to about 42.degree. Brix, the concentration of frozen concentrate available in retail stores. During this processing step, essence and fresh juice are often added to enhance flavor and aroma that are lost when processing the fresh juice initially for shipping and storage.
The processes used to concentrate fruit and vegetable juices often involve steps which detract from the characteristics desired by consumers or have other drawbacks. Juices contain spoilage microorganisms that must be essentially destroyed to achieve adequate storage time (shelf life). These spoilage microorganisms are generally destroyed by pasteurization at temperatures of about 62.degree. C. for 30 minutes or higher temperatures for shorter periods of time. Unfortunately, this process also volatilizes or destroys the aroma and flavor components that are of low molecular weight (30 to 155) and are easily affected at temperatures above 40.degree. C.
Furthermore, fruits and vegetables contain enzymes which can cause off-aroma, off-flavor, loss of color and other undesirable characteristics. Pectinesterase, one undesirable enzyme for example, must be inactivated if one wants to keep the desirable characteristic body and cloud in juices such as orange and grapefruit juice. Temperatures often higher than those needed to pasteurize are needed to inactivate pectinesterase. Consequently, loss of flavor and aroma components is further compounded.
U.S. Pat. No. 4,643,902 (U.S. '902), which is incorporated herein by reference, teaches a process for avoiding many disadvantages of conventional juice processing. It uses ultrafiltration to preferentially pass a UF permeate containing flavor and aroma components while retaining spoilage microorganisms in a UF retentate. The UF retentate is then treated to inactivate, as by heating, a sufficient number of spoilage microorganisms to inhibit spoilage of the juice under storage conditions. The treated UF retentate is then recombined with the UF permeate that contains the flavor and aroma components to provide a food juice suitable for storage but having retained flavor and aroma components.
U.S. '902 avoids much of the volatilization of flavor and aroma components and subsequent need to recover some of the aqueous essence associated with conventional thermal evaporative concentration such as is done by thermally accelerated short time evaporation (TASTE) units for concentrating frozen orange juice. It avoids the entrainment of flavor and aroma components in ice crystals separated from the freeze concentrate during freeze concentration and the oxidative degradation associated with freeze concentration and sublimation concentration.
The process of U.S. '902 first separates pulp and other solids from the juice to a level sufficient for efficient ultrafiltration. It then employs an ultrafiltration stage to preferentially pass a UF permeate containing the lower molecular weight volatile flavor components of fruits and vegetables ranging in molecular weight from about 30 to 155 and other molecules such as sugar and amino acids while retaining larger molecules. The membranes are stated to be tighter than the bacteriological filters which have a pore size of less than 1 nanometer and, thus, retain spoilage compounds such as bacteria, yeasts, molds, fungi and the like as well as undesirable enzymes such as oxidase and pectinesterase, proteins, pectin, and oils.
The UF retentate is passed to an inactivation stage in which undesirable components are inactivated. U.S. '902 does not teach any critical limitations for this stage. It states that the methods employed can vary with spoilage microorganisms being inactivated or destroyed by heat, chemical treatment, desiccation, UV radiation, x-rays and the like. For foods, heating is the preferred method of inactivation.
The UF permeate is fed to a reverse osmosis (RO) unit to concentrate the flavor and aroma components as a RO retentate. The RO retentate, free of most of the spoilage microorganisms which remained in the UF retentate, can be recombined with the inactivated UF retentate to make a storage stable product, for example, a 50.degree. Brix orange juice product capable of storage at about -4.degree. C. for at least 12 months without spoiling.
Nevertheless, it has been found that flavor and aroma losses still occur and the final product quality is not so good as desired. This is now hypothesized as being due to two factors. It is felt that some flavor and aroma components are retained in the UF retentate even though the pore size (about 20,000 to 100,000 MWCO) theoretically should allow all such components (molecular weight of about 30 to 155) to pass through. Additionally, it is felt that the product is adversely affected if the processing time for the UF retentate is too long even if the process time is at low temperatures.
By using UF membranes sized to allow the flavor and aroma components to pass through as suggested in U.S. '902, a gel layer forms on the surface of the membrane reducing the effective pore size and resulting in retention of the smaller aroma and flavor components in the UF retentate. Also, the membranes tend to become plugged, particularly at high concentrations of soluble and insoluble components. As the membrane becomes plugged, the processing time for the UF retentate increases and product quality declines. By using a tighter UF membrane, plugging can be minimized but flavor and aroma components may be retained in the UF retentate instead of passing through into the UF permeate as desired.
Furthermore, some of the flavor and aroma components that are fed to the food juice RO concentrators taught in the art pass through into the RO permeate which is discarded.
The RO system of U.S. '902 has further limitations since final concentration depends on the operating pressure needed to overcome the osmotic pressure of the concentrated juice, the viscosity of the concentrate and fouling caused by pectin and other ingredients. Thus, a juice concentrate of about 25.degree. to 30.degree. Brix is typically produced. By employing membranes operable at higher pressures (1500 pounds per square inch gauge), a clarified orange juice, for example, can be concentrated to about 42.degree. Brix.
U.S. Pat. No. 3,617,550 discloses a process for concentrating a feed solution by forcing it through a series of high rejection membranes, discarding or recycling the permeate and then further concentrating the retentate using a series of low rejection membranes where the osmotic pressure of the retentate exceeds the working pressure of the low rejection membranes. Preferably, the permeate from the low rejection membranes is recycled to the feed to the high rejection membranes. The process enables production of concentrates having osmotic pressures of several thousand pounds per square inch gauge (psig), which is above the working pressure of the reverse osmosis membranes taught. Orange juice concentrate, for example, with an osmotic pressure of three to four thousand psig would be about 60.degree. to 65.degree. Brix.